Silica (SiO2) films formed by a vacuum process such as a CVD method are abundantly used as an interlayer dielectric of semiconductor devices in large scale integrated circuits (LSI), and the like. In order to form a semiconductor interlayer dielectric with a more uniform film thickness, a coating-type insulating film, called an SOG (spin on glass), which contains an alkoxysilane hydrolyzate as a major component, has been used in recent years. Moreover, along with high integration of LSI, an interlayer dielectric with a low relative dielectric constant in which an organic silica sol such as MSQ (methylsilsesquioxane) is used as a major component has also been developed (U.S. Pat. Nos. 6,235,101, 6,413,647, and 6,495,264).
The organic silica sol can be cured by a dehydration condensation reaction of silanol groups contained in the sol by heating at 350 to 500° C., to produce an insulating film having a low relative dielectric constant, mechanical properties, and chemical resistance suitable for semiconductor devices. However, since this reaction of the organic silica sol is a solid phase reaction, the dehydration condensation proceeds only with difficulty due to diffusion restriction, requiring a long heating time of at least about 30 minutes, and usually one hour. A batch heating furnace which can heat many (usually 50 to 150) wafers at once for a long period of time is used for processing a spin-on interlayer dielectric with a low relative dielectric constant. However, the semiconductor device which mainly needs an interlayer dielectric with a low relative dielectric constant is in the logic device field, in which the device wiring manufacturing process is shifting to a sheet-feed process to produce wafers one by one in a short time. Since ASICs and custom ICs which are in the mainstream of the logic device are produced in high-mix low-volume production, a sheet-feed process is mainly used in order to promote freedom of the manufacturing process.
As a method for curing a composition for forming an interlayer dielectric with a low relative dielectric constant, which uses organic silica sol as a major component in a short time, and improving the strength, a method using electron beams has been proposed (U.S. Pat. No. 6,204,201 and European Patent No. 1,122,770). The method is characterized by positively decomposing and activating the organic groups in the organic silica film and introducing new crosslinking of Si—CHx-Si and the like, in addition to causing a silanol condensation reaction to occur by simultaneous heating and electron beam irradiation. Use of electron beams makes it possible to produce films with only minimal moisture absorption and having excellent mechanical properties, usually within five minutes, and to process wafers by a sheet-feed process. On the other hand, there is a concern that electron beam irradiation may accumulate electric charges which damage the transistor structure in LSIs. Thus, there are pros and cons in curing the composition for forming an interlayer dielectric with a low relative dielectric constant using electron beams (E. Mickler et al. Proceedings of the International Interconnect Technology Conference, p 190, 2004, Miyajima, et al. Proceedings of the International Interconnect Technology Conference, p 222, 2004).
Besides the electron beams, ultraviolet radiation may be another candidate to be used in a method of curing a composition for forming an interlayer dielectric with a low relative dielectric constant which uses organic silica sol as a major component in a short time. The point of discussion is once changed to technologies other than the interlayer dielectric for LSIs. A photo sol-gel technology of gelling silica sol in which the condensation reaction of silanol and alkoxide is accelerated by adding a photoacid generator or a photobase generator, which generates an acid or a base by ultraviolet irradiation to silica sol and alkoxysilane, is known and applied to optical waveguide formation and the like (JP-A-2000-109695, etc.) A silica film produced by curing using such a photoacid generator or a photobase generator generally contains a large amount of silanol residue and is highly hygroscopic. As a result, the film has a high relative dielectric constant. The moisture absorption by silanol residue can be reduced by heating the gel obtained by ultraviolet irradiation at about 250 to 500° C. for a period longer than a specific period of time (usually 30 minutes or more). This method is no more than the above-mentioned method of curing the silica film by heating. In addition, in the composition containing such a photoacid generator or a photobase generator, the photoacid generator or photobase generator and the acids and bases produced therefrom act as charged carriers which cause problems such as impairing insulation and deteriorating wiring metals. Such a composition, therefore, cannot satisfy qualities of the insulation film of semiconductor devices for LSIs in which high insulation reliability is required.
Due to the excellent transparency to ultraviolet radiation, the siloxane compound has also been studied as a main skeleton of F2 photoresist which uses ultraviolet radiation at a wavelength of 157 nm. Although siloxane is used as a backbone, the technology basically applies the principle of chemical amplification photoresist with which KrF and ArF light sources are also used. That is, ultraviolet radiation is used not for promoting the crosslinking reaction of the silica sol, but for causing the photoacid generator to generate an acid. The acid cleaves chemical bonds and produces a functional group such as a carboxylic acid which is readily dissolved in a basic developer. This technology does not promote the crosslinking reaction of the silica sol using ultraviolet radiation.
Since the surface of an organic silica film cured with heat, electron beams, or the like is hydrophobic, such a surface is treated with ultraviolet radiation with an objective of improving the hydrophobic properties (U.S. Pat. No. 6,383,913, JP-A-63-248710, JP-A-63-289939, JP-B-8-29932, JP-A-2001-110802, and etc.). According to these methods, the pole surface of an organic silica film is oxidized with ozone generated by irradiating the film with ultraviolet rays in the air, whereby the hydrophobic surface is reformed to the hydrophilic surface which is more reactive with silanol and the like. The reforming is mainly carried out in order to increase adhesiveness with a film which is formed on the upper layer.
As described above, technology of applying a polysiloxane resin solution or an organic silica sol solution to a substrate and applying ultraviolet radiation after forming a film has been widely studied. However, few technologies are reported involving the use of ultraviolet radiation for curing organic silica sol in order to form an interlayer dielectric of semiconductor devices for LSI. A few examples are disclosed in JP-A-3-30427, JP-A-1-194980, WO 03/025994, and US-A-2004/0058090.
JP-A-3-30427 discloses a technology of obtaining a silicon dioxide film at a low temperature by applying a solution of tetraalkoxysilane (for example, tetraethoxysilane (TEOS)) in collodion to a semiconductor substrate and applying ultraviolet radiation under a nitrogen atmosphere. This technology is characterized by fixing highly volatile TEOS by collodion and promoting decomposition of collodion and the dehydration-condensation of TEOS by ultraviolet radiation. JP-A-1-194980 discloses a technology in which an organosiloxane resin is applied to a substrate and subjected to ultraviolet radiation with a main wavelength of 254 nm with heating at 200° C. or less, the surface of the organosiloxane film is oxidized by ozone produced by application of ultraviolet radiation, and the organosiloxane film is heated at 400° C. or more, particularly about 900° C., to obtain a densified silicon dioxide film.
On the other hand, WO 03/025994 and US-A-2004/0058090 disclose a method of curing HSQ (hydrogenated silsesquioxane) or MSQ by ultraviolet radiation. In this method, active oxygen (for example, ozone) generated in the system by irradiating HSQ or HSQ/MSQ condensate in the presence of oxygen promotes oxidation of Si-H in the HSQ to form a silica film with many SiO2 bonds. These documents outline that the technique is effective also for MSQ and the presence of oxygen is more effective for curing, and that formation of SiO2 bonds by active oxygen is thought to be the main mechanism of the crosslinking reaction. Thus, it is difficult for the other curing methods to form SiO2 bonds in such a short period of time. The use of ultraviolet radiation is one of the technical features of this method. However, due to the increased number of SiO2 bonds, which results in a film with high modulus of elasticity and high hardness on the one hand, the silica film produced by this method has a problem of producing a film with high hydrophilicity which increases the moisture absorption and relative dielectric constant. Generally, a film with high hygroscopicity is easily damaged by RIE (reactive ion etching) at the time of processing the interlayer dielectric for semiconductor devices and has poor resistance to a wet cleaning liquid. This tendency is particularly remarkable in the case of an interlayer dielectric with a low relative dielectric constant of k=2.5 or less in which the insulation film itself has a porous structure. Therefore,    (a) an organic silica sol composition curable in a short time, not containing ionic substances such as a photoacid generator, a photobase generator, and a photosensitizer, or a source of a charge carrier or a corrosive compound,    (b) a method for curing an organic silica film which can be processed by a sheet-feed process without damaging the transistor structure,    (c) an organic silica film having low hygroscopicity and high hydrophobicity, and    (d) an organic silica film having mechanical properties resistive to formation of a copper damascene structure are desired for forming an interlayer dielectric with a low relative dielectric constant used in semiconductor devices for LSI.
The organic silica sol composition for forming an insulation film with a low relative dielectric constant for semiconductor devices is usually formulated so as to exhibit a high modulus of elasticity after heat curing, taking the yield when producing a dynamic stress such as CMP (chemical mechanical polishing), packaging, and the like into consideration (for example, U.S. Pat. No. 6,495,264). Specifically, an absolute crosslink density of the silica film can be increased by increasing the amount of tetra-functional silane compounds (hereinafter referred to from time to time as “Q component”) in the organic silica to 40 mol % or more. The crosslink density is increased by increasing the amount of the Q component and a film with a high modulus of elasticity and hardness can be obtained. However, it is difficult to completely react the crosslinking part (silanol) possessed by the Q component. Use of too large an amount of the Q component increases silanol residue after heat curing, resulting in hydrophilic and high hygroscopic film. In order to compensate this weak point, a method of reducing the absolute amount of silanols in the sol by producing a sol with a high degree of condensation by carrying out the co-condensation reaction with an alkoxysilane having a hydrophobic group such as methyltrialkoxysilane usually in the presence of a basic catalyst (such as ammonia and tetraalkylhydroxyammonium) (U.S. Pat. No. 6,413,647), and a method of additionally processing the high condensation sol by a hydrophobic treatment (JP-A-2004-59737 and JP-A-2004-149714) have been proposed. The organic silica sol containing a large amount of such a Q component still does not necessarily exhibit sufficient RIE resistance and wet cleaning liquid resistance.
The inventors of the invention have previously proposed a method for producing a polymer by co-condensation of a polysiloxane derived from a silane monomer containing a hydrolyzable group and a polycarbosilane by reacting the hydrolyzable group-containing silane monomer in the presence of the polycarbosilane, and a method for producing a hybrid polymer film having a low relative dielectric constant and exhibiting no phase separation in the film by using a film-forming composition containing this polymer (WO 2005/068538). However, the hybrid polymer film may be inferior in modulus of elasticity and anti-crack properties in the film thickness beyond 1 micrometer to a silica film formed using a composition containing the Q component as a main raw material.